Replace 'Poissonian' hazard curve calculator with 'Generalized' hazard curve calculator

[Damiano Monelli]

Pull request: https://github.com/gem/oq-hazardlib/pull/142 replaces the 'Poissonian' hazard curve calculator with a generalized hazard calculator that does not rely on the Poissonian assumption. The new calculator requires a temporal occurrence model to be an attribute of a seismic source. This means updating the nrml schema so that, if not specified, the temporal occurrence model associated to a source is 'Poissonian'. This information needs to be passed when a 'hazardlib' source is created.

[Michele Simionato (michele-simionato)]

This is a backward-incompatible change which adds a new attribute to the rupture objects. Therefore it is impossible to reuse old ruptures from a calculation run before the change in hazardlib. Notice also that a migration of the ruptures stored in the db would be extra-slow.

[Michele Simionato (michele-simionato)]

https://github.com/gem/oq-engine/pull/1359